This section provides background information related to the present disclosure which is not necessarily prior art.
As a result of environmental and other concerns, plastic containers (more specifically polyester and even more specifically polyethylene terephthalate (PET) containers) are now being used more than ever to package numerous commodities previously supplied in glass containers. Manufacturers and fillers, as well as consumers, have recognized that PET containers are lightweight, inexpensive, recyclable and manufacturable in large quantities.
Blow-molded plastic containers have become commonplace in packaging numerous commodities. PET is a crystallizable polymer, meaning that it is available in an amorphous form or a semi-crystalline form. The ability of a PET container to maintain its material integrity relates to the percentage of the PET container in crystalline form, also known as the “crystallinity” of the PET container. The following equation defines the percentage of crystallinity as a volume fraction:
      %    ⁢                  ⁢    Crystallinity    =            (                        ρ          -                      ρ            a                                                ρ            c                    -                      ρ            a                              )        ×    100  where ρ is the density of the PET material, ρa is the density of pure amorphous PET material (1.333 g/cc), and ρc is the density of pure crystalline material (1.455 g/cc). Once a container has been blown, a commodity may be filled into the container.
In some cases, a container is formed in a plurality of blow molding steps. For instance, a primary form of the container is initially blow molded inside a primary mold, and then a secondary form of the container is subsequently blow molded inside a secondary mold. The size of the primary and secondary molds is controlled such that the primary form can fit inside the secondary mold. For instance, the cavity inside the primary mold is often approximately equal to the size of the cavity inside the secondary mold such that the secondary mold can accommodate the primary form of the container.
Also, the primary form of the container often shrinks slightly after being molded in the primary mold. The size of the primary and secondary molds can, thus, be further controlled to account for this shrink back of the primary form of the container. Also, in some embodiments, suction is applied inside the primary form after being formed in the primary mold to facilitate and control shrink back. Likewise, in some embodiments, the primary form is reheated after being blow molded in the primary mold to facilitate and control the amount of shrink back.